Lightmaps are created by VRAD each time a map is compiled. They are static and cannot change, though it is possible to switch lightmap 'pages' on and off (see Naming Lights).

Scale

The lightmap scale of a face defines the resolution of its lightmap. The default scale of 16 makes each lightmap pixel ("luxel") 16 units across, while a lightmap scale of 1 makes one luxel equal one unit. The Hammer Face Edit Dialog is used to change the value per-face.

Lowering scale will make lightmap shadows sharper, but leads to larger map filesize, slightly slower rendering and exponentially slower compiles (that at very low scales start to eat up shocking amounts of system memory).

Lightmap scales of 4, 16, and 64.

When generating lightmaps, VRAD does not create penumbras. The effect can be done by placing multiple lights near each other to simulate a non-point source of light, and/or increasing lightmap scale (which is why the relatively large value of 16 is the default).

Note:Each compiled brush polygon can host a maximum of 31x31 luxels. VBSP will chop faces to get extra luxels on if it needs to. That is also the reason for the limited displacement luxel density

Tip:Fog in reality increases atmospheric scattering, making all shadows a lot more diffuse. Look at light_environment's sun spread angle

Optimization

Optimising lightmap scales.

Lightmap optimization can be done by eye with the aid of a compiled map: faces with low lighting contrast can have their scales increased, and vice versa faces with high contrast should be considered for lower scale. Just beware of setting a face's scale so high that it ends up a 'blob' of light that doesn't blend with its neighbors (use mat_fullbright 2 to spot this).

Remember to take into account the size of a face, too. Moving a very large face even one point up or down can have a huge impact. Don't be afraid to split the face up if that helps, or to drop a bit of detail from huge floor or wall faces.

Hammer view

Clicking the camera control in the top-left of a 3D view in Hammer provides the option of "3D Lightmap Grid". This view textures each brush surface with a grid that represents its lightmap scale (shown in the last section).

Tool brushes are textured like any other in this mode, despite being invisible in-game, so switch them all off from their auto-visgroup.